In any bleaching pulp process, filtrate management is an important factor in the overall economy or cost of operation of the process. The water which is used in the process requires both access to a suitable source and treatment of the effluent prior to discharge.
In an effort to reduce the water demand of the process, it is desirable to recycle as much of the effluent as possible. This practice cannot be used with processes utilizing chlorine or multiple steps of chlorine dioxide, since the effluents produced by these processes contain large amounts of chlorides produced by the by-products of such chemicals. Thus, recycling these effluents would cause a build-up of chlorides which, in turn, would cause either corrosion of processing equipment or the use of expensive materials of construction for such equipment. In addition, such effluents require substantial treatment before they can be discharged from the mill, thus requiring further expenditures for equipment and treatment chemicals.
The use of either the conventional CEDED or OC/DED processes results in a significant disposal problem with regard to the effluents produced from the washing steps due to the high levels of chloride-containing compounds found therein. As noted above, these streams cannot be recycled, and are preferably treated before discharge into the environment. Recycling of effluent could be used to decrease the amount of water used, but then the process equipment may be subject to increased corrosion rates due to the increased chloride levels of the recycled effluents.
U.S. Pat. No. 5,164,043 discloses an environmentally improved chlorine-free process for bleaching pulp with oxygen ("O"), ozone ("Z") and then chlorine dioxide ("D") or a peroxide ("P"). A modified oxygen ("O.sub.m ") stage followed by a modified ozone ("Z.sub.m ") stage is a preferred delignification sequence. The O.sub.m stage is conducted by reducing the consistency of the pulp to less than about 5%, substantially uniformly combining the pulp with alkaline material, increasing the consistency of the pulp to obtain the desired amount of alkaline material uniformly distributed thereon, and subjecting the pulp to high consistency oxygen delignification. The Z.sub.m stage is conducted by acidifying the pulp, adding a chelating agent, increasing the consistency of the pulp to greater than 20%, and turbulently mixing the pulp at the high consistency with ozone in a dynamic reaction chamber.
The ozone delignified pulp may be subjected to an alkaline extraction stage ("E") and is then brightened by the D or P stage. The use of an O.sub.m Z.sub.m ED process, for example, results in the formation of only a minimal amount of chlorinated material in the wash water, which water can be safely discharged, i.e., sewered, within most environmental protection standards. Alternately, this effluent may be treated by reverse osmosis to provide an even cleaner filtrate that may be recycled to previous bleaching stages as shown for further use without the build-up of chlorides.
When a D bleaching stage is desired, steps may be taken to reduce the demand for chlorine dioxide. An E.sub.o, E.sub.p or E.sub.op step allows the pulp to achieve greater levels of brightness although additional expense is incurred by the use of additional sodium hydroxide and oxygen and/or peroxide in this step. Also, there are known industry procedures for preparing chlorine dioxide whereby residual chlorine levels are minimized (e.g., the R8 process vs. the R3 process). These reduced chlorine level chemicals are preferred for use in the D stage to reduce the chloride levels of the wash water effluent.
Instead of O.sub.m Z.sub.m ED, one may use the O.sub.m EP process of the invention to obtain additional substantial advantages over the prior art in that no chlorinated compounds whatsoever are produced. This enables all of the effluent to be recycled without experiencing the problems of chloride build-up in the process wash water streams.
As shown in FIG. 4 of the '043 patent, the bleach plant filtrates are recycled countercurrently so that cleaner filtrates are sent back to wash pulp in the earlier (i.e., dirtier) part of the plant in order to achieve a closed or semi-closed filtrate system.
It has now been found that the effluent from the washer downstream of the ozone reactor becomes acidic, primarily due to the relatively low pH conditions of the pulp in the ozone reactor. During typical continuous operation of the Z.sub.m stage, the washer effluent achieves a pH of about 3 to 4 due to the countercurrent flow of alkaline E-stage filtrate. When this washer effluent is recycled to the washer upstream of the ozone stage, the pH of the wash water in that washer drops, calcium, barium and other metals desorb from the pulp and salts of divalent cations such as calcium and barium, and in particular, calcium and barium oxalates, precipitate from the wash water. It has been found that this precipitation generally occurs in the washer, although it can occur in downstream process lines and equipment, such as in the acidification step or the ozone reactor, where it causes operability problems. The extent of scaling can be sufficiently large to cause plugging or blocking of the equipment and require shutdown of the process to remove the precipitated salts. To resolve this problem, it is necessary to reduce the concentration of the divalent cation in this part of the process, or to not recycle the stream that contains it.
It is generally known that concentrations of undesirable ions in a filtrate or effluent stream can be reduced by purging and sewering all or a portion of the stream. Such a practice is not desirable, however, because it increases the water demand for the plant as well as the costs for handling the effluent which is to be discharged from the plant. In addition, this practice would require treatment of the purged stream before it could be properly discharged from the plant. It is also possible to use chelants in sufficient amounts to retain these salts in solution to avoid precipitation, but these additives would be relatively expensive.
Accordingly, what is needed is a method for preventing or controlling precipitation of salts from the wash water effluents or filtrates which are recycled in order to avoid forming substantial amounts of salt scale in process equipment, but without purging or discharging the effluents or filtrates which contain such salts. The present invention provides a simple, yet effective, method for resolving this problem.